fscrypto: require write access to mount to set encryption policy

From: Eric Biggers <[email protected]>

commit ba63f23d69a3a10e7e527a02702023da68ef8a6d upstream. [Please apply to 4.4-stable. Note: this was already backported, but only to ext4; it was missed that it should go to f2fs as well. This is needed to make xfstest generic/395 pass on f2fs.]

Since setting an encryption policy requires writing metadata to the filesystem, it should be guarded by mnt_want_write/mnt_drop_write. Otherwise, a user could cause a write to a frozen or readonly filesystem. This was handled correctly by f2fs but not by ext4. Make fscrypt_process_policy() handle it rather than relying on the filesystem to get it right.

---
 fs/f2fs/file.c | 6 ++++++
 1 file changed, 6 insertions(+)

index 4b449d263333..01eed94b01ea 100644
@@ -1541,12 +1541,18 @@ static int f2fs_ioc_set_encryption_policy(struct file *filp, unsigned long arg)
 				sizeof(policy)))
 		return -EFAULT;
 
+	err = mnt_want_write_file(filp);
+	if (err)
+		return err;
+
 	mutex_lock(&inode->i_mutex);
 
 	err = f2fs_process_policy(&policy, inode);
 
 	mutex_unlock(&inode->i_mutex);
 
+	mnt_drop_write_file(filp);
+
 	return err;
 #else
 	return -EOPNOTSUPP;

2.15.0.rc0.271.g36b669edcc-goog

Good catch, thanks for the patch.

greg k-h